A major change after 17.4 version was in the name of the API. Also, please note that to ensure the security, we also change the license signatures and encryption time to time. Which is why we always recommend to use the latest version of the API.
Furthermore, you can keep using the older version of the API for as long as you want/need without upgrading or renewing the license file. You only need to renew or purchase new license if you want to use the latest or higher version of the API which is not supported by your existing license.
In your case, you can use your older license with 17.4 version of the API for backward compatibility and to avail new features and updates, you can implement new code using latest version and the latest license you have.
Please feel free to let us know in case you need more information.